Piezoelectric composition and piezoelectric device

ABSTRACT

The piezoelectric composition is represented by the following Chemical Formula (1):
 
 x [Bi m FeO 3 ]- y [Ba m TiO 3 ]- z [Sr m TiO 3 ]  (1)
 
wherein 0.5≤x≤0.8, 0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and 0.96≤m≤1.04.

TECHNICAL FIELD

The present invention relates to a piezoelectric composition and a piezoelectric device.

BACKGROUND

A perovskite-type metal oxide is known as a common piezoelectric composition. The constitution of a perovskite-type metal oxide is represented by ABO₃. A perovskite-type piezoelectric composition is, for example, lead zirconate titanate (Pb(Zr, Ti)O₃). The curie temperature (T_(c)) of lead zirconate titanate (PZT) is high, and the piezoelectric constant (d₃₃) of PZT is large. However, PZT does harm to the environment or the human body due to containing lead as an element at A sites. The piezoelectric composition not containing lead is required in view of influence on the environment or the human body.

An example of the piezoelectric composition not containing lead is bismuth ferrite (BiFeO₃) described in the following Non Patent Literature 1. The Tc of bismuth ferrite (BFO) is high, and BFO exhibits large spontaneous polarization. However, in the case of BFO alone, enough piezoelectric performance (for example, d₃₃) is not obtained due to the anisotropy being high, the leakage current being large and the like.

Therefore, a piezoelectric composition the Tc of which is high and the d₃₃ of which is large is required. A binary compound composed of barium titanate and bismuth ferrite is disclosed in the following Non Patent Literature 2. A ternary compound composed of barium titanate, bismuth ferrite and a composite oxide such as bismuth magnesate titanate is disclosed in Japanese Unexamined Patent Publication 2013-191751.

-   [Non Patent literature 1] Tadej Rojac et al., “Strong ferroelectric     domain-wall pinning in BiFeO3 ceramics”, JOURNAL OF APPLIED PHYSICS,     108, 074107, 2010. -   [Non Patent literature 2] Zhenyong Cen et al., “Effect of sintering     temperature on microstructure and piezoelectric properties of     Pb-free BiFeO3-BaTiO3 ceramics in the composition range of large     BiFeO3 concentrations”, J Electroceram, 31, p. 15-20, 2013.

SUMMARY Problem to be Solved by the Invention

A piezoelectric composition described in Japanese Unexamined Patent Publication 2013-191751 is greatly distorted when an electric field high enough is impressed thereon. However, it is difficult for a piezoelectric composition described in Japanese Unexamined Patent Publication 2013-191751 to have a piezoelectric constant large enough after a poling process is performed thereon. Furthermore, when a poling process is performed on a piezoelectric composition described in Non Patent Literature 2, the d₃₃ of the piezoelectric composition after the poling process is around 130 pC/N, and is small.

Therefore, the present invention has been completed in view of the above circumstances, and an object of the present invention is to provide a piezoelectric composition the d₃₃ of which is large and the specific resistance of which is large and a piezoelectric device using the piezoelectric composition.

Means for Solving the Problem

A piezoelectric composition according to one aspect of the present invention is represented by the following Chemical Formula (1): x[Bi_(m)FeO₃]-y[Ba_(m)TiO₃]-z[Sr_(m)TiO₃]  (1) wherein 0.5≤x≤0.8, 0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and 0.96≤m≤1.04.

A piezoelectric device according to one aspect of the present invention comprises the above piezoelectric composition.

Effects of Invention

According to the present invention, a piezoelectric composition the d₃₃ of which is large and the specific resistance of which is large and a piezoelectric device using the piezoelectric composition are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a piezoelectric device according to one embodiment of the present invention.

FIG. 2 is a schematic view illustrating the constitutions of piezoelectric compositions in Examples and Comparative Examples.

DETAILED DESCRIPTION

Preferred embodiments of the present invention will be described in detail hereinafter with proper reference to the drawings. In the drawings, the identical or equivalent components are indicated with the identical reference sign. When a description overlaps, the description is omitted. The present invention is not limited to the following embodiments.

As shown in FIG. 1, a piezoelectric device 10 according to this embodiment comprises a pair of electrodes 4 a and 4 b and a piezoelectric body 2 between the pair of electrodes 4 a and 4 b. That is, an electrode 4 a overlaps one surface of the piezoelectric body 2, and a different electrode 4 b overlaps the other surface of the piezoelectric body 2.

A piezoelectric composition contained in the piezoelectric body 2 is represented by the following Chemical Formula (1). The piezoelectric body 2 may consist of only the piezoelectric composition represented by the following Chemical Formula (1). The piezoelectric body 2 may contain other ingredients in addition to the piezoelectric composition represented by the following Chemical Formula (1). The piezoelectric composition represented by the following Chemical Formula (1) may be a powder, or may be a sintered body. x[Bi_(m)FeO₃]-y[Ba_(m)TiO₃]-z[Sr_(m)TiO₃]  (1) [In Formula (1), 0.5≤x≤0.8, 0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and 0.96≤m≤1.04].

The piezoelectric composition represented by the above Chemical Formula (1) may have a perovskite-type crystal structure. The piezoelectric composition represented by the above Chemical Formula (1) may be a composite oxide represented by the following Chemical Formula (2). The piezoelectric body 2 may consist of only the piezoelectric composition represented by the following Chemical Formula (2). The piezoelectric body 2 may contain other ingredients in addition to the piezoelectric composition represented by the following Chemical Formula (2). (Bi_(x)Ba_(y)Sr_(z))_(m)(Fe_(x)Ti_((y+z)))O₃  (2) [In Formula (2), 0.5≤x≤0.8, 0.02≤y≤0.4, 0.02≤z≤0.2, x+y+z=1, and 0.96≤m≤1.04].

A portion of the piezoelectric composition may be a phase consisting of Bi_(m)FeO₃. A portion of the piezoelectric composition may be a phase consisting of Ba_(m)TiO₃. A portion of the piezoelectric composition may be a phase consisting of Sr_(m)TiO₃. A portion of the piezoelectric composition may be a solid solution of at least two oxides selected from the group consisting of Bi_(m)FeO₃, Ba_(m)TiO₃ and Sr_(m)TiO₃.

The present inventors consider that a reason the d₃₃ and the specific resistance of the above piezoelectric composition are large is as follows. In a process of sintering perovskite-type BiFeO₃ to form the piezoelectric body 2, Bi is likely to volatilize. Also in a process of forming a thin film containing BiFeO₃, Bi is likely to volatilize. For example, when a thin film is formed by a sputtering method, Bi is unlikely to be incorporated into the thin film in a growth process. Therefore, in conventional BiFeO₃, which does not contain Sr, Bi is likely to be deficient and vacancies are likely to occur at A sites of BiFeO₃. That is, in conventional BiFeO₃, defects at A sites are likely to occur. Meanwhile, a piezoelectric composition according to this embodiment contains Sr, which is unlikely to volatilize. Therefore, vacancies at A sites caused by the volatilization of Bi are replaced with Sr, and the defects at A sites are unlikely to occur. Consequently, the specific resistance of the piezoelectric composition according to this embodiment is larger than the specific resistance of conventional BiFeO₃. Furthermore, the piezoelectric composition according to this embodiment is formed from rhombohedral Bi_(m)FeO₃, tetragonal Ba_(m)TiO₃ and cubic Sr_(m)TiO₃. Therefore, polarization rotation becomes likely to occur near the boundaries of the three phases of Bi_(m)FeO₃, Ba_(m)TiO₃ and Sr_(m)TiO₃. Consequently, the d₃₃ of the piezoelectric composition according to this embodiment is larger than the d₃₃ of conventional BiFeO₃. In addition, the reason the d₃₃ and the specific resistance of the piezoelectric composition according to this embodiment are large is not limited to the above reason.

In the above Chemical Formula (1), the x satisfies 0.5≤x≤0.8. When the x is less than 0.5, the crystal structure of the piezoelectric composition shifts to a cubic crystal, and the d₃₃ of the piezoelectric composition is likely to become small. When the x is more than 0.8, the crystal structure of the piezoelectric composition shifts to a rhombohedral crystal, polarization reversal is unlikely to occur, the d₃₃ of the piezoelectric composition becomes small, many defects at A sites are caused by the volatilization of Bi, and the specific resistance is likely to decrease. It is preferable that the x satisfy 0.55≤x≤0.75. Since the x is in the above numerical value range, the d₃₃ and the specific resistance of the piezoelectric composition are likely to become large.

In the above Chemical Formula (1), y satisfies 0.02≤y≤0.4. When the y is less than 0.02, the crystal structure of a piezoelectric composition shifts to a rhombohedral crystal, the polarization reversal is unlikely to occur, and the d₃₃ of a piezoelectric composition becomes small, many defects at A sites are caused by the volatilization of Bi, and the specific resistance is likely to decrease. When the y is more than 0.4, the crystal structure of the piezoelectric composition shifts to a cubic crystal, and the d₃₃ of the piezoelectric composition is likely to become small. It is preferable that the y satisfy 0.15≤y≤0.33. Since the y is in the above numerical value range, the d₃₃ and the specific resistance of the piezoelectric composition are likely to become large.

In the above Chemical Formula (1), z satisfies 0.02≤z≤0.2. When the z is less than 0.02, the defects at A sites caused by the volatilization of Bi are not fully replaced with Sr. Consequently, the specific resistance is likely to decrease. When the z is more than 0.2, the crystal structure of the piezoelectric composition shifts to a cubic crystal, and the d₃₃ of the piezoelectric composition is likely to become small. It is preferable that the z satisfy 0.02≤z≤0.1. Since the z is in the above numerical value range, the d₃₃ and the specific resistance of the piezoelectric composition are likely to become large.

In the above Chemical Formula (1), the m satisfies 0.96≤m≤1.04. When the m is less than 0.96, Bi, Ba and Sr, which are at A sites, decrease, defects at A sites increase, and the specific resistance is likely to decrease. When the m is more than 1.04, heterogeneous phases containing any of Bi, Ba and Sr are formed in grain boundaries, and the d₃₃ of the piezoelectric composition is likely to become small. It is preferable that the m satisfy 0.98≤m≤1.02. Since the m is in the above numerical value range, the d₃₃ and the specific resistance of the piezoelectric composition are likely to become large.

The above Chemical Formula (2) is represented as A_(m)BO₃. The m is the ratio [A]/[B] of the number [A] of all atoms occupying A sites to the number [B] of all atoms occupying B sites. When the m is out of the above numerical value range, the m, namely the [A]/[B], greatly deviates from a stoichiometric ratio of 1, and therefore defects in the piezoelectric composition are likely to increase and heterogeneous phases are likely to increase in grain boundaries, thereby the d₃₃ and the specific resistance of the piezoelectric composition are likely to become small.

The piezoelectric composition may contain other elements than the elements contained in the above Chemical Formula (1) as impurities or minute amounts of additives in the form of compounds or simple substances. Examples of such a compound include oxides of Na, Al, Si, P, K, Fe, Cu, Zn, Hf, Ta, or W. When the piezoelectric composition contains these oxides and the like, it is preferable that the total content of oxides in the piezoelectric composition be 0.3% by mass or less of the whole piezoelectric composition in terms of oxides of the elements. That is, it is preferable that the main ingredient, specifically 99.7% by mass or more of the whole piezoelectric composition, have the constitution represented by Chemical Formula (1). In this case, the piezoelectric composition substantially has the constitution represented by Chemical Formula (1).

The constitution of the piezoelectric composition can be measured, for example, by an X-ray fluorescence analysis (XRF method).

Since the piezoelectric device 10 comprises the piezoelectric body 2 containing the piezoelectric composition represented by the above Chemical Formula (1), it is excellent in piezoelectric characteristics. The piezoelectric body 2 may be a sintered body containing the above piezoelectric composition, or may be a thin film containing the above piezoelectric composition.

The potential difference between a pair of electrodes 4 a and 4 b may be, for example, 0.1 to 2.0 kV/mm. In a piezoelectric device using conventional BiFeO₃, as long as the potential difference between the electrodes of the piezoelectric device is not a high voltage of 5.0 kV/mm or more, enough piezoelectric characteristics are not obtained. Meanwhile, in the piezoelectric device 10 according to this embodiment, even if the potential difference between the electrodes 4 a and 4 b is a low voltage in the above numerical value range or a high voltage higher than the above numerical value range, enough piezoelectric characteristics can be obtained.

Then, an example of a method for manufacturing the piezoelectric device 10 according to this embodiment will be described. The method for manufacturing the piezoelectric device 10 comprises: a mixing step of granulating a raw material powder of the piezoelectric body 2 containing the above piezoelectric composition; a sintering step of press-forming this raw material powder to form a formed body and sintering the formed body to produce a sintered body; and a polarization step of performing a poling process on the sintered body to obtain the piezoelectric device 10. Steps will be described specifically hereinafter.

In a mixing step, starting raw materials for preparing the piezoelectric composition are first provided. As the starting raw materials, oxides of elements that constitute the piezoelectric composition represented by the above Chemical Formula (1) or compounds that become these oxides after sintering (carbonates, hydroxides, oxalates, nitrates and the like) can be used. As specific starting raw materials, a Bi (bismuth) compound, a Fe (iron) compound, a Ba (barium) compound, a Ti (titanium) compound, a Sr (strontium) compound and the like may be used. These starting raw materials are blended at a molar ratio or a mass ratio such that the piezoelectric composition represented by the above Chemical Formula (1) is formed after sintering, and wet mixing is conducted by a ball mill or the like.

The Bi compound may be bismuth oxide (Bi₂O₃), bismuth nitrate (Bi(NO₃)₃) or the like. The Fe compound may be iron oxide (Fe₂O₃), iron chloride (FeCl₃), iron nitrate (Fe(NO₃)₃) or the like. The Ba compound may be barium oxide (BaO), barium carbonate (BaCO₃), barium oxalate (C₂BaO₄), barium acetate ((CH₃COO)₂Ba), barium nitrate (Ba(NO₃)₂), barium sulfate (BaSO₄), barium titanate (BaTiO₃) or the like. The Ti compound may be titanium oxide (TiO₂) or the like. The Sr compound may be strontium carbonate (SrCO₃) or the like.

Then, the mixed raw material obtained by wet mixing is temporarily formed into a temporarily formed body, which is then calcined. A calcined body containing the above piezoelectric composition is obtained by this calcination. It is preferable that the calcination temperature be 600 to 900° C., and it is preferable that the calcination time be around 1 to 16 hours. When the calcination temperature is too low, a chemical reaction tends not to proceed fully in the temporarily formed body. When the calcination temperature is too high, a temporarily formed body begins to sinter, and subsequent pulverization therefore tends to become difficult. Calcination may be performed in the air atmosphere, and may be performed in an atmosphere in which the partial pressure of oxygen is higher than that in the air, or a pure oxygen atmosphere. Furthermore, the wet-mixed starting raw material may be calcined without being temporarily formed.

Then, the obtained calcined body is slurried and pulverized (wet ground) by a ball mill or the like, and thereafter a fine powder is obtained by drying the slurry. A binder is added to the obtained fine powder if needed, and the raw material powder is granulated. It is preferable to use water, an alcohol such as ethanol, a mixed solvent of water and ethanol, or the like as a solvent for slurrying a calcined body. Binders added to the fine powder include commonly used organic binders such as polyvinyl alcohol, polyvinyl alcohol to which a dispersing agent is added, ethyl cellulose, and the like.

In a sintering step, a formed body is formed by press-forming the granulated raw material powder. A load at the time of press-forming may be adjusted, for example, to 1.0 to 3.5 MPa.

Then, binder removal treatment is provided to the obtained formed body. It is preferable that the binder removal treatment be performed at a temperature of 400 to 800° C. for around 2 to 4 hours. The binder removal treatment may be performed in the air atmosphere, or may be performed in an atmosphere in which the partial pressure of oxygen is higher than that in the air, or a pure oxygen atmosphere.

A sintered body containing the piezoelectric composition represented by the above Chemical Formula (1) is obtained by sintering the formed body after the binder removal treatment. The sintering temperature may be about 900 to 1100° C., and the sintering time may be adjusted to around 2 to 20 hours. The binder removal treatment and the sintering of the formed body may be performed successively, or may be performed separately.

The surface of the obtained sintered body is polished if needed, and the sintered body is processed such that the sintered body is cut to form the desired shape of a piezoelectric body 2. A pair of electrodes 4 a and 4 b is formed on both surfaces of the processed sintered body. The electrodes 4 a and 4 b may be formed by applying and baking an electrode paste, or may be formed by vapor deposition or sputtering film formation.

A piezoelectric device 10 according to this embodiment can be obtained by impressing a polarization electric field on the sintered body on which the electrodes are formed and performing a poling process. The conditions of the poling process may be properly determined depending on the constitution of the piezoelectric composition that the sintered body contains. The conditions of the poling process may be, for example, as follows. The sintered body may be immersed into silicone oil at 25 to 200° C. The time to impress a polarization electric field may be 5 to 60 minutes. The magnitude of a polarization electric field may be 1.2 or more times greater than that of the coercive electric field of the sintered body.

The uses of a piezoelectric composition according to this embodiment are various. The piezoelectric composition may be applied, for example, to a radiator, a resonator, an actuator, a motor or a sensor. The specific use of the piezoelectric composition may be, for example, a SAW filter, a BAW filter, a piezoelectric microphone, a head assembly, a hard disk drive, a printer head, an ink jet printer device, an ultrasonic washing machine, an ultrasonic motor, an atomizer oscillator, a fish finder, a shocking sensor, an ultrasonic diagnostic device, a waste toner sensor, a gyro sensor, a buzzer, a transformer, or a lighter.

As mentioned above, preferable embodiments of the present invention were described, but the present invention is not necessarily limited to the embodiments mentioned above. Various modifications of the present invention are possible as long as the modifications do not deviate from aims of the present invention, and these modification examples are included in the present invention.

EXAMPLES

The present invention will be described in detail by using Examples and Comparative Example hereinafter. However, the present invention is not limited to the following Examples at all.

Example 1

A piezoelectric device was manufactured by a method illustrated below. As raw materials of the piezoelectric composition, raw material powders of bismuth oxide (Bi₂O₃), iron oxide (Fe₂O₃), barium carbonate (BaCO₃), strontium carbonate (SrCO₃), and titanium oxide (TiO₂) were provided. These raw material powders were weighed and blended, and the mixed raw material was prepared such that a sintered body after the final sintering became a piezoelectric composition represented by the following Chemical Formula (3). x[Bi_(m)FeO₃]-y[Ba_(m)TiO₃]-z[Sr_(m)TiO₃]  (3) In the above Chemical Formula (3), x=0.58, y=0.4, z=0.02, and m=1.00.

Then, the prepared mixed raw material and pure water were mixed with Zr balls by a ball mill for 10 hours to obtain slurry. This slurry was fully dried, thereafter press-formed and calcined at 800° C. to obtain a calcined body. Next, the calcined body was pulverized by the ball mill and thereafter dried, a proper amount of PVA (polyvinyl alcohol) was added thereto as a binder, and the mixture was granulated. Around 3 g of the obtained granulated powder was placed into a metal mold of 20 mm in length by 20 mm in width and formed under a load of 3.2 MPa by using a uniaxial press molding machine.

The molded sample was heat-treated to remove the binder therefrom and thereafter finally sintered at 950 to 1100° C. for 4 hours to obtain a sintered body.

The obtained sintered body was flattened to a thickness of 0.5 mm with a double-side lapping machine and thereafter cut to a size of 16 mm in length by 16 mm in width with a dicing saw. Subsequently, electrodes 4 a and 4 b were formed on both surfaces of the sintered body by using a vacuum evaporator. The electrodes 4 a and 4 b were composed of 1.5-μm Ag layers. The sizes of the electrodes were 15 mm by 15 mm.

Thereafter, a poling process in which 1.5 to 2 times as strong an electric field as the coercive electric field was impressed on the sintered body on which the electrodes were formed in a silicone oil bath at a temperature of 120° C. for 15 minutes was performed to obtain a piezoelectric device 10 having the same configuration as FIG. 1.

[Analysis of Constitution]

The constitution of the sintered body in Example 1 was analyzed by an X-ray fluorescence analysis (XRF method). Consequently, it has been confirmed that the constitution of the sintered body in Example 1 is represented by the above Chemical Formula (3).

[Measurement of d₃₃]

The d₃₃ (unit: pC/N) of the sintered body (piezoelectric composition) in Example 1 before the forming of temporary electrodes was measured by using a d₃₃ meter. The above d₃₃ meter measures d₃₃ by the Berlincourt method based on JIS R 1696. In the Berlincourt method, the d₃₃ is measured by utilizing a piezoelectric positive effect at the time when vibration is given to a piezoelectric composition. Therefore, in the Berlincourt method, there is no influence of electrostriction differently from a measuring method in which a piezoelectric reverse effect at the time when an electric field is impressed on a piezoelectric composition is utilized, and the original d₃₃ of a piezoelectric composition is obtained. The d₃₃ in Example 1 is illustrated in Table 1. The d₃₃ is good when it is 150 pC/N or more.

[Measurement of Specific Resistance]

Direct-current voltage was impressed between the electrode 4 a and the electrode 4 b in the piezoelectric device in Example 1, and the specific resistance (unit: Ω·cm) between the electrode 4 a and the electrode 4 b was measured. The direct-current voltage was 40 V. The specific resistance in Example 1 is illustrated in Table 1. The specific resistance is good when it is 1.0×10⁹ Ω·cm or more.

Examples 2 to 11 and Comparative Examples 1 to 6

Respective piezoelectric devices in Examples 2 to 11 and Comparative Examples 1 to 6 were individually manufactured by the same method as in Example 1 except that the ratios of powder raw materials blended were altered such that x, y, z, and m in the above Chemical Formula (3) became values illustrated in the following Table 1 or 2 when mixed raw materials were prepared. The respective constitutions of the sintered bodies in Examples 2 to 11 and Comparative Examples 1 to 6 were analyzed by the same method as in Example 1. Consequently, it has been confirmed that the respective constitutions of the sintered bodies in Examples 2 to 11 and Comparative Examples 1 to 6 are the constitutions illustrated in Table 1 or 2. The d₃₃s of Examples 2 to 11 and Comparative Examples 1 to 6 were measured individually by the same method as in Example 1. The d₃₃s of Examples 2 to 11 and Comparative Examples 1 to 6 are illustrated in Table 1 or 2. The specific resistances in Examples 2 to 11 and the Comparative Examples 1 to 6 were measured individually by the same method as in Example 1. The specific resistances in Examples 2 to 11 and Comparative Examples 1 to 6 are illustrated in Table 1 or 2. In Comparative Example 5, since the sintered body had broken when a poling process was performed, the d₃₃ and the specific resistance were not measured.

[Table 1]

TABLE 1 Specific resistance BiFeO₃ BaTiO₃ SrTiO₃ A/B d₃₃ (pC/N) (Ω · cm) x y z m — — Example 1 0.58 0.4 0.02 1.00 201 2.5 × 10¹² Example 2 0.8 0.18 0.02 1.00 211 8.9 × 10¹¹ Example 3 0.78 0.02 0.2 1.00 168 7.6 × 10¹¹ Example 4 0.5 0.3 0.2 1.00 152 3.1 × 10¹² Example 5 0.65 0.33 0.02 1.00 232 3.1 × 10¹² Example 6 0.75 0.23 0.02 1.00 222 1.0 × 10¹² Example 7 0.7 0.15 0.15 1.00 190 7.6 × 10¹¹ Example 8 0.55 0.3 0.15 1.00 216 5.5 × 10¹¹ Example 9 0.65 0.25 0.1 1.00 219 2.2 × 10¹² Comparative 0.6 0.4 0 1.00 195 2.1 × 10⁸  Example 1 Comparative 0.8 0.2 0 1.00 210 1.2 × 10⁸  Example 2 Comparative 0.8 0 0.2 1.00 135 9.3 × 10¹⁰ Example 3 Comparative 0.48 0.32 0.2 1.00 121 1.2 × 10¹² Example 4

TABLE 2 Specific resistance BiFeO₃ BaTiO₃ SrTiO₃ A/B d₃₃ (pC/N) (Ω · cm) x y z m — — Example 9 0.65 0.25 0.1 1.00 219 2.2 × 10¹² Example 10 0.69 0.4 0.1 0.96 175 9.8 × 10⁹  Example 11 0.69 0.4 0.1 1.04 170 4.4 × 10¹¹ Comparative 0.5 0.3 0.2 0.94 — — Example 5 Comparative 0.65 0.33 0.02 1.06 145 6.8 × 10¹¹ Example 6

FIG. 2 is a schematic view illustrating the constitutions of the piezoelectric compositions in Examples and Comparative Examples. BF means BiFeO₃ in FIG. 2. BT means BaTiO₃. ST means SrTiO₃. The constitutions of the piezoelectric compositions are illustrated by coordinates (x, y, z) in FIG. 2. A number appended to a round mark is the number of Example, and a number appended to a triangle mark is the number of Comparative Example. That is, the coordinates of a round mark indicate the constitution of a piezoelectric composition in Example, and the coordinates of a triangle mark indicate the constitution of a piezoelectric composition in Comparative Example. The coordinates (x, y, z) of Example 1 are (0.58, 0.4, 0.02). The coordinates (x, y, z) of Example 2 are (0.8, 0.18, 0.02). The coordinates (x, y, z) of Example 3 are (0.78, 0.02, 0.2). The coordinates (x, y, z) of Example 4 are (0.5, 0.3, 0.2). A range surrounded by dotted lines connecting the coordinates of Examples 1 to 4 is defined as a range A. The range A is in the range of the constitutions of the piezoelectric compositions according to the present invention. The coordinates (x, y, z) of Example 5 are (0.65, 0.33, 0.02). The coordinates (x, y, z) of Example 6 are (0.75, 0.23, 0.02). The coordinates (x, y, z) of Example 7 are (0.7, 0.15, 0.15). The coordinates (x, y, z) of Example 8 are (0.55, 0.3, 0.15). A range surrounded by dashed lines connecting the coordinates of Examples 5 to 8 is defined as a range B. The piezoelectric compositions belonging to the range B are superior to the piezoelectric compositions belonging to the above range A in the point that at least either of the d₃₃ and the specific resistance is likely to become large.

From the above experimental results, it has been confirmed that all the Examples are in the range of the piezoelectric composition represented by the above Chemical Formula (1). Meanwhile, it has been confirmed that all the Comparative Examples are out of the range of the piezoelectric composition represented by the above Chemical Formula (1).

As illustrated in Tables 1 and 2, the d₃₃s of all the Examples were 150 pC/N or more, and the specific resistances of all the Examples were 1.0×10⁹ Ω·cm or more. Meanwhile, there was no Comparative Examples the d₃₃s of which were 150 pC/N or more and the specific resistances of which were 1.0×10⁹ Ω·cm or more. It has been confirmed that a piezoelectric composition the d₃₃ of which is large and the specific resistance of which is large and a piezoelectric device using the piezoelectric composition are provided according to the present invention.

REFERENCE SIGNS LIST

2: piezoelectric body; 4 a, 4 b: electrode; 10: piezoelectric device. 

What is claimed is:
 1. A piezoelectric composition represented by a following Chemical Formula (1): x[Bi_(m)FeO₃]-y[Ba_(m)TiO₃]-z[Sr_(m)TiO₃]  (1) where 0.55≤x≤0.75, 0.15≤y≤0.33, 0.02≤z≤0.1, x+y+z=1, and 0.96≤m≤1.04.
 2. A piezoelectric device comprising the piezoelectric composition according to claim
 1. 